Not exactly sure what the confusion is. The patch and task are clear to me and solve the issue.
Moreover traditionally, the user agent is the name of the software, not the origin. So this could also deserve another comment about why we should replace the specific software name with a fixed origin. Premising that I agree that the current user agent may be re-discussed, to include more information, like, "Phorge", and the version of Phorge.
Actually no, user agents for automated tooling often includes some sort of contact email or website or link to a page describing the bot/tool. I think this should be treated like that.
Also, in case, maybe probably nice to have a getUserAgent(__CLASS__) method in parent class PhutilAuthAdapter()